Log Message:
Update mksh to 56c.

R56c is a bugfix-only release everyone must upgrade to:

- [komh] Remove redundant OS/2-specific code, clean up others
- [komh, tg] Fix drive-qualified (absolute and relative) DOS-style path support
  in realpath functionality, partially other places
- [tg] Don't substitute ${ENV:-~/.mkshrc} result again
- [tg] Improve OS/2 $PATH (et al.) handling, drive-relative paths
- [tg] Add MKSH_ENVDIR compile-time option for Jehanne and Plan 9
- [tg] Limit nesting when parsing malformed code (Debian #878947)
- [tg] Update wcwidth data with bugfixed script (still Unicode 10; resulting
  values are identical to glibc git master for extant chars)
- [Dr. Werner Fink] Raise some time limits in the testsuite
- [Shamar] Add support for the Jehanne operating system
- [komh] Set stdin to text mode before executing child processes on OS/2
- [komh] Pass arguments via a resonse file if executing a child fails
- [Dr. Werner Fink] Early locale tracking as a compile-time option
- [tg] Fix regressions introduced with new fast character classes